class User
  include Mongoid::Document

  field :name

  # has_and_belongs_to_many :favorited_shops, :stored_as => :array, :inverse_of => :favoritors, :class_name => "Shop"
  has_and_belongs_to_many :followed_shops, :stored_as => :array, :inverse_of => :followers, :class_name => "Shop"
  has_one :shop  
end

class Shop
  include Mongoid::Document
  field :title

  # This is work fine.
  # has_and_belongs_to_many :favoritors, :stored_as => :array, :inverse_of => :favorited_shops, :class_name => "User"
  
  # This can not working
  has_and_belongs_to_many :followers, :stored_as => :array, :inverse_of => :followed_shops, :class_name => "User"
  
  belongs_to :user
end

ruby> u = User.first
ruby> u.shop = Shop.new
or
ruby> u.create_shop(...)

Mongoid undefined method `map' for <User:....>